Ras regulation of cyclin D1 promoter

EGFR inhibition in glioma cells modulates Rho signaling to inhibit cell motility and invasion and cooperates with temozolomide to reduce cell growth

Enforced EGFR activation upon gene amplification and/or mutation is a common hallmark of malignant glioma. Small molecule EGFR tyrosine kinase inhibitors, such as erlotinib (Tarceva), have shown some activity in a subset of glioma patients in recent trials, although the reported data on the cellular basis of glioma cell responsiveness to these compounds have been contradictory. Here we have used a panel of human glioma cell lines, including cells with amplified or mutant EGFR, to further characterize the cellular effects of EGFR inhibition with erlotinib. Dose-response and cellular growth assays indicate that erlotinib reduces cell proliferation in all tested cell lines without inducing cytotoxic effects. Flow cytometric analyses confirm that EGFR inhibition does not induce apoptosis in glioma cells, leading to cell cycle arrest in G₁. Interestingly, erlotinib also prevents spontaneous multicellular tumour spheroid growth in U87MG cells and cooperates with sub-optimal doses of temozolomide (TMZ) to reduce multicellular tumour spheroid growth. This cooperation appears to be schedule-dependent, since pre-treatment with erlotinib protects against TMZ-induced cytotoxicity whereas concomitant treatment results in a cooperative effect. Cell cycle arrest in erlotinib-treated cells is associated with an inhibition of ERK and Akt signaling, resulting in cyclin D1 downregulation, an increase in p27^kip1 levels and pRB hypophosphorylation. Interestingly, EGFR inhibition also perturbs Rho GTPase signaling and cellular morphology, leading to Rho/ROCK-dependent formation of actin stress fibres and the inhibition of glioma cell motility and invasion.

Transcriptional tools: Small molecules for modulating CBP KIX-dependent transcriptional activators

Previously it was demonstrated that amphipathic isoxazolidines are able to functionally replace the transcriptional activation domains of endogenous transcriptional activators. In addition, in vitro binding studies suggested that a key binding partner of these molecules is the CREB Binding Protein (CBP), more specifically the KIX domain within this protein. Here we show that CBP plays an essential role in the ability of isoxazolidine transcriptional activation domains to activate transcription in cells. Consistent with this model, isoxazolidines are able to function as competitive inhibitors of the activators MLL and Jun, both of which utilize a binding interaction with KIX to up-regulate transcription. Further, modification of the N2 side chain produced three analogs with enhanced potency against Jun-mediated transcription, although increased cytotoxicity was also observed. Collectively these small KIX-binding molecules will be useful tools for dissecting the role of the KIX domain in a variety of pathological processes.

Junctional music that the nucleus hears: cell-cell contact signaling and the modulation of gene activity

Cell-cell junctions continue to capture the interest of cell and developmental biologists, with an emerging area being the molecular means by which junctional signals relate to gene activity in the nucleus. Although complexities often arise in determining the direct versus indirect nature of such signal transduction, it is clear that such pathways are essential for the function of tissues and that alterations may contribute to many pathological outcomes. This review assesses a variety of cell-cell junction-to-nuclear signaling pathways, and outlines interesting areas for further study.

Inhibition of tumor growth and metastasis by non-small cell lung cancer cells transfected with cyclin D1-targeted siRNA

To observe whether cyclin D1 siRNA-mediated inhibition of cyclin D1 represents a promising antigrowth and antimetastatic strategy for cancer gene therapy, particularly for non-small cell lung cancers. To stably transfect the A549 cell line with a cyclin D1-targeted siRNA to downregulate cyclin D1 expression and observe the effects on protein expression, and tumor growth in vitro and in vivo. Expression of cyclin D1-targeted siRNA resulted in a decrease in cyclin D1, MMP-2, RhoA, and Rac1 protein levels, as detected by Western blot and immunofluorescence studies. Transfected cells also exhibited a marked decrease in the rate of cell growth, and decreased invasive capacity, compared to cells transduced with a scrambled siRNA plasmid and untransduced A549 cells. siRNA-mediated inhibition of cyclin D1 expression represents a promising antigrowth and antimetastatic strategy for cancer gene therapy, particularly for non-small cell lung cancers. It is the reason for inhibiting tumor growth so that cyclin D1 siRNA can inhibit the cell cycle progression. In addition, the mechanism of inhibiting tumor metastasis was related to the decrease in the expression of MMP-2, RhoA, and Rac1 after cyclin D1 was decreased by cyclin D1 siRNA.

Tight junctions and the regulation of gene expression

Cell adhesion is a key regulator of cell differentiation. Cell interactions with neighboring cells and the extracellular matrix regulate gene expression, cell proliferation, polarity and apoptosis. Apical cell-cell junctions participate in these processes using different types of proteins, some of them exhibit nuclear and junctional localization and are called NACos for Nuclear Adhesion Complexes. Tight junctions are one type of such cell-cell junctions and several signaling complexes have been identified to associate with them. In general, expression of tight junction components suppresses proliferation to allow differentiation in a coordinated manner with adherens junctions and extracellular matrix adhesion. These tight junction components have been shown to affect several signaling and transcriptional pathways, and changes in the expression of tight junction proteins are associated with several disease conditions, such as cancer. Here, we will review how tight junction proteins participate in the regulation of gene expression and cell proliferation, as well as how they are regulated themselves by different mechanisms involved in gene expression and cell differentiation.

Protein kinase C alpha signaling inhibits cyclin D1 translation in intestinal epithelial cells

Cyclin D1 is a key regulator of cell proliferation, acting as a mitogen sensor and linking extracellular signaling to the cell cycle machinery. Strict control of cyclin D1 levels is critical for maintenance of tissue homeostasis. We have reported previously that protein kinase C alpha (PKCalpha), a negative regulator of cell growth in the intestinal epithelium, promotes rapid down-regulation of cyclin D1 (Frey, M. R., Clark, J. A., Leontieva, O., Uronis, J. M., Black, A. R., and Black, J. D. (2000) J. Cell Biol. 151, 763-778). The current study explores the mechanisms underlying PKCalpha-induced loss of cyclin D1 protein in non-transformed intestinal epithelial cells. Our findings exclude several mechanisms previously implicated in down-regulation of cyclin D1 during cell cycle exit/differentiation, including alterations in cyclin D1 mRNA expression and protein turnover. Instead, we identify PKCalpha as a novel repressor of cyclin D1 translation, acting at the level of cap-dependent initiation. Inhibition of cyclin D1 translation initiation is mediated by PKCalpha-induced hypophosphorylation/activation of the translational suppressor 4E-BP1, association of 4E-BP1 with the mRNA cap-binding protein eIF4E, and sequestration of cyclin D1 mRNA in 4E-BP1-associated complexes. Together, these post-transcriptional effects ensure rapid disappearance of the potent mitogenic molecule cyclin D1 during PKCalpha-induced cell cycle withdrawal in the intestinal epithelium.

Regulation of PCNA and cyclin D1 expression and epithelial morphogenesis by the ZO-1-regulated transcription factor ZONAB/DbpA

The tight junction protein ZO-1 inhibits G1/S-phase transition by cytoplasmic sequestration of a complex formed by CDK4 and the transcription factor ZONAB. Canine ZONAB is the homologue of human DbpA, an E2F target gene that is overexpressed in different carcinomas. Since the ZONAB target genes that are involved in G1/S-phase transition are unknown, we employed the mammary epithelial cell line MCF-10A and cDNA arrays to screen for such genes. We identified genes encoding cell cycle and replication proteins whose expression was altered due to increased ZONAB expression. For proliferative cell nuclear antigen and cyclin D1 genes, we show that increased mRNA levels resulted in increased protein levels and we identified ZONAB-responsive elements in their promoters by using different approaches, including chromatin immunoprecipitation assays. RNA interference and overexpression of ZONAB affected the proliferation of both MCF-10A and MDCK cells as well as the differentiation of MDCK cells into polarized cysts in three-dimensional cultures. These results indicate that ZONAB regulates the transcription of genes that are important for G1/S-phase progression and links tight junctions to the transcriptional control of key cell cycle regulators and epithelial cell differentiation.

Ras and Myc can drive oncogenic cell proliferation through individual D-cyclins

D-type cyclins serve as cell cycle recipients of several oncogenic pathways. The specific sequences of the promoters of the cyclin D genes are thought to render particular D-cyclins responsive to specific oncogenic pathways. For instance, the Ras oncogene was postulated to signal through cyclin D1, while Myc can impact the cell cycle machinery by transcriptionally upregulating cyclin D2. In the current study we engineered mouse fibroblasts to express only cyclin D1, only D2, or only D3. These 'single-cyclin' cells allowed us to rigorously test the ability of cyclin D1, D2, or D3, when expressed on their own, to serve as recipients of the Ras- and Myc-driven oncogenic pathways. We found that each of the D-cyclins was sufficient to drive oncogenic proliferation of mouse fibroblasts. This, together with our recent observations that cells lacking all three D-cyclins show greatly reduced susceptibility to the oncogenic action of Ras and Myc, reveals that the Ras and Myc oncogenes can impact the core cell cycle machinery through all three D-cyclins.

Cyclin D1 represses p300 transactivation through a cyclin-dependent kinase-independent mechanism

Cyclin D1 encodes a regulatory subunit, which with its cyclin-dependent kinase (Cdk)-binding partner forms a holoenzyme that phosphorylates and inactivates the retinoblastoma protein. In addition to its Cdk binding-dependent functions, cyclin D1 regulates cellular differentiation in part by modifying several transcription factors and nuclear receptors. The molecular mechanism through which cyclin D1 regulates the function of transcription factors involved in cellular differentiation remains to be clarified. The histone acetyltransferase protein p300 is a co-integrator required for regulation of multiple transcription factors. Here we show that cyclin D1 physically interacts with p300 and represses p300 transactivation. We demonstrated further that the interaction of the two proteins occurs at the peroxisome proliferator-activated receptor gamma-responsive element of the lipoprotein lipase promoter in the context of the local chromatin structure. We have mapped the domains in p300 and cyclin D1 involved in this interaction. The bromo domain and cysteine- and histidine-rich domains of p300 were required for repression by cyclin D1. Cyclin D1 repression of p300 was independent of the Cdk- and retinoblastoma protein-binding domains of cyclin D1. Cyclin D1 inhibits histone acetyltransferase activity of p300 in vitro. Microarray analysis identified a signature of genes repressed by cyclin D1 and induced by p300 that promotes cellular differentiation and induces cell cycle arrest. Together, our results suggest that cyclin D1 plays an important role in cellular proliferation and differentiation through regulation of p300.

Destabilization of cyclin D1 message plays a critical role in cell cycle exit upon mitogen withdrawal

Cyclin D1 is critical for entry into, continuation of, and exit from the cell division cycle. Mitogen stimulation of quiescent cells induces cyclin D1 expression in a transcription-dependent manner. In actively cycling cells, on the other hand, fluctuation of cyclin D1 protein levels through the cell cycle is post-transcriptionally regulated. Cyclin D1 is expressed at low levels during S phase to allow efficient DNA synthesis, and induced to high levels in G2 phase through Ras activity to commit the cells to continuing cell cycle progression. Once induced in G2 phase, cyclin D1 expression becomes Ras independent through the next G1 phase, where it promotes G1/S transition. When mitogenic signaling is abrogated, however, cyclin D1 fails to increase during G2 phase and the cell becomes arrested in the next G1 phase. In this way, the expression levels of cyclin D1 in G2 phase determine the fate of the next cell cycle. Despite its importance of the mechanism of cyclin D1 suppression upon mitogen withdrawal is unknown. Using both quantitative fluorescence microscopy and biochemical analyses, we have found that, upon serum deprivation, cyclin D1 mRNA is downmodulated without any decline in its rate of transcription. Furthermore, cyclin D1 mRNA half-life becomes shorter when serum is removed. These results demonstrate that cyclin D1 message destabilization plays a critical role in cyclin D1 suppression during G2 phase of serum-deprived cultures, and therefore in the withdrawal from the cell cycle.

Cooperative regulation of the cell division cycle by the protein kinases RAF and AKT

The RAS-activated RAF-->MEK-->extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3'-kinase (PI3'-kinase)-->PDK1-->AKT signaling pathways are believed to cooperate to promote the proliferation of normal cells and the aberrant proliferation of cancer cells. To explore the mechanisms that underlie such cooperation, we have derived cells harboring conditionally active, steroid hormone-regulated forms of RAF and AKT. These cells permit the assessment of the biological and biochemical effects of activation of these protein kinases either alone or in combination with one another. Under conditions where activation of neither RAF nor AKT alone promoted S-phase progression, coactivation of both kinases elicited a robust proliferative response. Moreover, under conditions where high-level activation of RAF induced G(1) cell cycle arrest, activation of AKT bypassed the arrest and promoted S-phase progression. At the level of the cell cycle machinery, RAF and AKT cooperated to induce cyclin D1 and repress p27(Kip1) expression. Repression of p27(Kip1) was accompanied by a dramatic reduction in KIP1 mRNA and was observed in primary mouse embryo fibroblasts derived from mice either lacking SKP2 or expressing a T187A mutated form of p27(Kip1). Consistent with these observations, pharmacological inhibition of MEK or PI3'-kinase inhibited the effects of activated RAS on the expression of p27(Kip1) in NIH 3T3 fibroblasts and in a panel of bona fide human pancreatic cancer cell lines. Furthermore, we demonstrated that AKT activation led to sustained activation of cyclin/cdk2 complexes that occurred concomitantly with the removal of RAF-induced p21(Cip1) from cyclin E/cdk2 complexes. Cumulatively, these data strongly suggest that the RAF-->MEK-->ERK and PI3'K-->PDK-->AKT signaling pathways can cooperate to promote G(0)-->G(1)-->S-phase cell cycle progression in both normal and cancer cells.

Minireview: Cyclin D1: normal and abnormal functions

Cyclin D1 encodes the regulatory subunit of a holoenzyme that phosphorylates and inactivates the retinoblastoma protein and promotes progression through the G1-S phase of the cell cycle. Amplification or overexpression of cyclin D1 plays pivotal roles in the development of a subset of human cancers including parathyroid adenoma, breast cancer, colon cancer, lymphoma, melanoma, and prostate cancer. Of the three D-type cyclins, each of which binds cyclin-dependent kinase (CDK), it is cyclin D1 overexpression that is predominantly associated with human tumorigenesis and cellular metastases. In recent years accumulating evidence suggests that in addition to its original description as a CDK-dependent regulator of the cell cycle, cyclin D1 also conveys cell cycle or CDK-independent functions. Cyclin D1 associates with, and regulates activity of, transcription factors, coactivators and corepressors that govern histone acetylation and chromatin remodeling proteins. The recent findings that cyclin D1 regulates cellular metabolism, fat cell differentiation and cellular migration have refocused attention on novel functions of cyclin D1 and their possible role in tumorigenesis. In this review, both the classic and novel functions of cyclin D1 are discussed with emphasis on the CDK-independent functions of cyclin D1.

Mammalian cells cycle without the D-type cyclin-dependent kinases Cdk4 and Cdk6

Cdk4 and Cdk6 are thought to be essential for initiation of the cell cycle in response to mitogenic stimuli. Previous studies have shown that Cdk4 is dispensable for proliferation in most cell types, an observation attributed to a putative compensatory role by Cdk6. Cdk6-null mice are viable and develop normally although hematopoiesis is slightly impaired. Embryos defective for Cdk4 and Cdk6 die during the late stages of embryonic development due to severe anemia. However, these embryos display normal organogenesis and most cell types proliferate normally. In vitro, embryonic fibroblasts lacking Cdk4 and Cdk6 proliferate and become immortal upon serial passage. Moreover, quiescent Cdk4/Cdk6-null cells respond to serum stimulation and enter S phase with normal kinetics although with lower efficiency. These results indicate that D-type cyclin-dependent kinases are not essential for cell cycle entry and suggest the existence of alternative mechanisms to initiate cell proliferation upon mitogenic stimulation.

IKKalpha regulates mitogenic signaling through transcriptional induction of cyclin D1 via Tcf

The Wnt/beta-catenin/Tcf and IkappaB/NF-kappaB cascades are independent pathways involved in cell cycle control, cellular differentiation, and inflammation. Constitutive Wnt/beta-catenin signaling occurs in certain cancers from mutation of components of the pathway and from activating growth factor receptors, including RON and MET. The resulting accumulation of cytoplasmic and nuclear beta-catenin interacts with the Tcf/LEF transcription factors to induce target genes. The IkappaB kinase complex (IKK) that phosphorylates IkappaB contains IKKalpha, IKKbeta, and IKKgamma. Here we show that the cyclin D1 gene functions as a point of convergence between the Wnt/beta-catenin and IkappaB pathways in mitogenic signaling. Mitogenic induction of G(1)-S phase progression and cyclin D1 expression was PI3K dependent, and cyclin D1(-/-) cells showed reduced PI3K-dependent S-phase entry. PI3K-dependent induction of cyclin D1 was blocked by inhibitors of PI3K/Akt/IkappaB/IKKalpha or beta-catenin signaling. A single Tcf site in the cyclin D1 promoter was required for induction by PI3K or IKKalpha. In IKKalpha(-/-) cells, mitogen-induced DNA synthesis, and expression of Tcf-responsive genes was reduced. Reintroduction of IKKalpha restored normal mitogen induction of cyclin D1 through a Tcf site. In IKKalpha(-/-) cells, beta-catenin phosphorylation was decreased and purified IKKalpha was sufficient for phosphorylation of beta-catenin through its N-terminus in vitro. Because IKKalpha but not IKKbeta induced cyclin D1 expression through Tcf activity, these studies indicate that the relative levels of IKKalpha and IKKbeta may alter their substrate and signaling specificities to regulate mitogen-induced DNA synthesis through distinct mechanisms.

Induction of p202, a modulator of apoptosis, during oncogenic transformation of NIH 3T3 cells by activated H-Ras (Q61L) contributes to cell survival

Previous studies have revealed that p202 (52 kDa), an interferon (IFN) and differentiation-inducible protein, negatively regulates cell proliferation and modulates cell survival. However, the role of p202 in transformed cells remains to be investigated. Here we report that constitutive expression of oncogenic H-Ras (Q61L) in NIH 3T3 cells, which resulted in cell transformation, was associated with increases in the steady-state levels of 202 RNA and protein. Interestingly, the increase in p202 levels in transformed cells correlated with increases in the activity of the transcription factor c-Jun/AP-1, which bound to the two potential AP-1 DNA binding sites (the AP-1CS1 and AP-1CS2) in the 5'-regulatory region of the 202 gene in gel mobility shift assays. Furthermore, the site-directed mutagenesis, coupled with promoter-reporter analyses, revealed that these two AP-1 DNA binding sites contribute to the regulation of the 202 gene in Ras transformed cells. Because treatment of transformed cells with a specific inhibitor of MEK (PD 98059) resulted in significant decreases in the levels of p202, these observations raise the possibility that in transformed cells Ras/Raf/MEK pathway regulates the transcriptional activation of the 202 gene. Significantly, decreases in the levels of p202 in Ras transformed NIH 3T3 cells under reduced serum conditions increased the susceptibility to apoptosis. Collectively, our observations support the idea that the transcriptional increases in the levels of p202 by oncogenic H-Ras in NIH 3T3 cells are needed for cell survival.

Post-transcriptional regulation of cyclin D1 expression during G2 phase

During continuous proliferation, cyclin D1 protein is induced to high levels in a Ras-dependent manner as cells progress from S phase to G2 phase. To understand the mechanism of the Ras-dependent cyclin D1 induction, cyclin D1 mRNA levels were determined by quantitative image analysis following fluorescent in situ hybridization. Although a slight increase in mRNA expression levels was detected during the S/G2 transition, this increase could not explain the more robust induction of cyclin D1 protein levels. This suggested the involvement of post-transcriptional regulation as a mechanism of cyclin D1 protein induction. To directly test this hypothesis, the cyclin D1 transcription rate was determined by run-on assays. The transcription rate of cyclin D1 stayed steady during the synchronous transition from S the G2 phase. We further demonstrated that cyclin D1 protein levels could increase during G2 phase in the absence of new mRNA synthesis. alpha-Amanitin, a transcription inhibitor, did not suppress cyclin D1 protein elevation as the cells progressed from S to G2 phase, even though the inhibitor was able to completely block cyclin D1 protein induction during reentry into the cell cycle from quiescence. The half life of cyclin D1 protein was shortest during S phase indicating that a change in protein stability might play a role in post-translational induction of cyclin D1 in G2 phase. These data indicate a fundamental difference in the regulation of cyclin D1 production during continuous cell cycle progression and re-initiation of the cell cycle.

Autocrine human growth hormone inhibits placental transforming growth factor-beta gene transcription to prevent apoptosis and allow cell cycle progression of human mammary carcinoma cells

Multiple cellular effects of human growth hormone (hGH) are mediated by an indirect mechanism requiring transcriptional activation of genes encoding protein effector molecules such as insulin-like growth factor-1. Such protein effector molecules then act directly to mediate the cellular functions of hGH. We report here that autocrine hGH production by mammary carcinoma cells specifically results in the transcriptional repression of the p53-regulated placental transforming growth factor-beta (PTGF-beta) gene. Transcriptional repression of the PTGF-beta gene does not require the p53-binding sites in the PTGF-beta promoter, and autocrine hGH also desensitized the response of the PTGF-beta promoter to p53 overexpression. Transcriptional repression of the PTGF-beta gene is accompanied by consequent decreases in its protein product, Smad-mediated transcription, and its cellular effects that include cell cycle arrest and apoptosis. PTGF-beta specifically inhibited the autocrine hGH-stimulated expression of cyclin D1 required for autocrine hGH-stimulated mammary carcinoma cell cycle progression. Thus, one mechanism by which autocrine hGH promotes an increase in mammary carcinoma cell number is by transcriptional repression of protein effector molecules that promote cell cycle arrest and apoptosis. Such transcriptional repression of negative regulatory factors, such as PTGF-beta, may also be requisite for direct stimulation of mammary carcinoma cell mitogenesis by hGH.